The present invention relates to control systems for molding machines, its particular application being to presses that are used to mold material that cures exothermically.
There are a variety of thermoset materials that are cured in the mold in an exothermic reaction. The reaction is typically initiated by heating the material while it is in the mold or just before it is charged into it. The reaction thus initiated liberates heat that causes the action to continue until curing has completed.
It is important in this type of molding that the mold not be opened too soon. If it is opened prematurely, the thermoset material must be scrapped because it cannot be used again for molding. Accordingly, operators are usually conservative and allow more than the minimum time required before opening the mold. However, competing considerations such as the rather high cost of molding-machine time indicate that this conservative approach can be wasteful if carried too far. The ideal solution, of course, is to open the mold as soon as the curing has been completed. It might be thought that if the exact curing time for a particular cavity and material part were known, the cycle time of the machine could be set, and then all parts made with that particular cavity and material molded would be produced with the minimum necessary cycle time. Unfortunately, material variations and drift in machine and ambient conditions cause curing times to vary widely, even between parts of the same type on the same day. Therefore, it has been necessary to set the cycle time at the upper limit of expected curing time and suffer the concomitant waste in machine time.
It is accordingly the object of the present invention to detect when curing has occurred in each cycle and direct the molding machine to open the mold shortly thereafter. Cycle times that are nearly as low as possible are thereby afforded while waste due to premature mold opening is avoided.